Phase selective track circuit apparatus

Abstract

A frequency selective filter is connected between the track transformer and phase selective unit at the receiving end of a coded phase selective track circuit to reject interfering signals of propulsion current frequency. The series L-C filter comprises a capacitor and a two-winding reactor coil with a resistor permanently connected in series between the two windings. Each winding is tapped to allow a selection of the filter inductance to match track circuit impedance, including impedance bonds and ballast resistance. Taps for short track circuits include the second winding and the resistor to improve signal to noise ratio. Energy for short interlocking track circuits is of a higher frequency, and a special tap on the first winding is selected to enable filter tuning at this other frequency for train detection since basic frequency is still used during code off-time for cab signal control.

Having thus described our invention, what we claim is: 
     
       1. For use in a phase selective track circuit for a railroad track section, said track circuit including a source of energy of selected frequency coupled to the rails at one end of the corresponding track section, a track coupling means connected to the rails at the other end of said section, and a phase selective means connected to said source and also selectively coupled through said track coupling means for receiving phase shifted energy through said rails when the section is unoccupied by a train, a tuning filter arrangement comprising, a. a reactor means having a first and a second winding, each winding having at least one tap lead between the end leads for selecting an inductance value different than the full winding inductance,   b. a capacitor having a preselected impedance value,   c. a resistor having a preselected resistance value,   d. said capacitor, said first winding, said resistor, and said second winding in order being connected in a series network and connected at the capacitor end to said track coupling means,   e. a selective connector lead connected for completing the coupling of said phase selective means to said track coupling means through said series network, 1. said connector lead being selectively connected to the tap or end lead of either winding for tuning the phase selective means path to substantially said selected frequency and for matching the rail circuit impedance to establish the phase relationship of the input signals to said phase selective means within a predetermined optimum range,   2. said connector lead being connected to the first winding tap or near end lead when the track circuit has greater than a preselected length and selectively to the second winding tap or end lead to include said resistor in the phase selective means coupling when the track circuit has less than said preselected length.     
     
     
       2. A phase selective track circuit arrangement, for a section of electrified railroad track, comprising in combination, a. a source of alternating current of a selected frequency different from propulsion energy frequency, coupled to the rails at one end of said track section for supplying operating energy to said track circuit,   b. a phase selective means coupled to said source and, when also coupled to said rails at the other end of said section, responsive to the reception of a track voltage signal having a predetermined phase relationship with said source voltage for registering the unoccupied condition of said section,   c. an L-C series filter network including first and second tapped reactor windings for adjusting the inductance of said network,   d. said filter network further including   
     
     
       1. a fixed resistor connecting said first and second tapped windings in series, and 2. a fixed capacitor connected in series to said windings and resistor circuit path and coupled to said rails at said other end of said section,   e. said phase selective means being coupled to said rails at said other end of said section through a selected tap on said first or second winding in accordance with the length of the track section for rejecting any signal of the propulsion energy frequency induced into the track circuit and for balancing the total series impedance of said track circuit arrangement to maintain said predetermined phase relationship of said track and source voltage signals received by said phase selective means when said section is unoccupied, 1. taps on said second winding being selected to include said resistor in said series network to reduce the sensitivity of said phase selective means only when the length of said track section is less than a predetermined distance, thus improving the signal to noise ratio of track circuit response.     
     
     
       3. A track circuit arrangement as defined in claim 2 which further includes, a. a code transmission means which alternately closes a first and a second contact at a periodic rate,   b. said code transmission means being connected for controlling the transmission of a pulse of operating energy to said rails from said selected frequency source each time said first contact closes,   c. said phase selective means being further responsive to the reception of coded energy within the frequency and phase limits to detect an unoccupied track section.   
     
     
       4. A track circuit arrangement as defined in claim 3 which further includes, a. a second source of alternating current having a frequency different from that of said first source and that of propulsion energy, for providing operating energy for short track circuits,   b. said code transmission means being connected for controlling the transmission of a second source pulse when said second contact closes for providing operating energy for train detection in short track circuits, 1. said first source being connected for providing a cab signal energy pulse when said first contact closes,     c. another tap on said first winding to provide a predetermined inductance adjustment to tune said filter network to said second source frequency,   d. said phase selective means being connected to a selected second winding tap and said capacitor connected to said first winding other tap to exclude response by said phase selective means to any but said second source frequency and to increase the sensitivity of said filter network to improve the signal to noise ratio to exclude harmonics of said propulsion frequency.   
     
     
       5. A track circuit arrangement as defined in claim 4 in which, a. said track section is insulated from each adjoining track section in the stretch, each associated pair of of insulated joints bypassed by an impedance bond network, of predetermined track circuit impedance, for providing a propulsion energy return circuit,   b. the first or second winding tap to tune said filter network and balance the track circuit equivalent impedance selected in accordance with the track section length, impedance bond impedance, rail and ballast impedances, the equivalent impedance of the source in use, and the effective impedance of the selected filter network.   
     
     
       6. Frequency selective filter apparatus, for a phase selective track circuit which includes the rails of a track section, a source of alternating current energy of selected frequency coupled for supplying track energy to said rails at one end of the corresponding track section, and phase selective means for detecting the occupancy condition of the section when coupled to said source and to said rails at the other end of said section to receive said track energy, comprising, a. a selectable inductance reactor comprising first and second windings, each tapped for adjusting the inductance of that winding,   b. a fixed resistor connecting said first and second winding in series,   c. a fixed capacitor connected in series with the windings and resistor circuit path to form an adjustable L-C circuit network with the capacitor end coupled to one of said rails at said other end,   d. said phase selective means being coupled to said rails through a selected tap on said first or second winding in accordance with the length of said track section, said resistor included with the L-C circuit portion used only when said section length is less than a preselected distance, for selecting the inductance of said reactor to match the impedance of the rail circuit and phase selective means for substantially excluding from said phase selective means induced energy of any frequency other than said selected frequency and for maintaining the phase relationship of signals supplied to said phase selective means within a predetermined range.   
     
     
       7. Filter apparatus as defined in claim 6 in which, a. the stretch of track including said corresponding section is electrified for train propulsion at a frequency different from said selected frequency but with harmonics closely spaced to said selected frequency,   b. said track energy is of a frequency preselected for relatively short track sections and is coded alternately on and off for transmitting pulses of track energy into said section rails,   c. said phase selective means is coupled to a tap on said second reactor winding for decreasing the sensitivity of said filter to exclude any response by said phase selective means to closely spaced harmonics of said track energy.   
     
     
       8. Filter apparatus as defined in claim 7 in which, a. cab signal energy having a frequency different from said preselected short section frequency and said propulsion frequency is supplied to said section rails during the off time of said track energy code pulses, and in which said L-C circuit also includes,   b. another first winding tab at a predetermined inductance adjustment to which said capacitor is selectively connected for changing the tuning response of said filter to exclude response by said phase selective means to cab signal energy.